QVSS: Fix to get the VSYNC interrupts to be more reasonable.  Addresses Issue #88.  From Mike Burke

The vertical sync interrupt generated in vc_svc() was too slow for Ultrix. The driver enables interrupts and then calls DELAY(20000), which I guess is 20ms. This translates to about 10000 simulated instructions, but tmxr_poll works out at ~75000 instructions.  I've added a new unit to simulate just the vsync interrupts activating every 8000 instructions.

This approach will completely disable the ability to idle while the QVSS device is enabled.